Oxygenate Conversion to Olefins with Dimerization and Metathesis

ABSTRACT

A processing scheme and system for enhanced light olefin production, particularly for increased relative yield of propylene, involves oxygenate conversion to olefins and subsequent oxygenate conversion effluent stream treatment including dimerization of ethylene to butene and metathesis of butenes and/or hexenes with ethylene. The processing scheme and system may further involve isomerization of at least a portion of 1-butene to 2-butene to produce additional propylene.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of copending U.S. application Ser.No. 11/643,604 filed Dec. 21, 2006, the contents of which are herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates generally to the conversion of oxygenates toolefins and, more particularly, to light olefins.

BACKGROUND OF THE INVENTION

A major portion of the worldwide petrochemical industry is in involvedwith the production of light olefin materials and their subsequent usein the production of numerous important chemical products viapolymerization, oligomerization, alkylation and the like well-knownchemical reactions. Light olefins include ethylene, propylene andcombinations thereof. These light olefins are essential building blocksfor the modern petrochemical and chemical industries. The major sourcefor these materials in present day refining is the stream cracking ofpetroleum feeds. For various reasons including geographical, economic,political and diminished supply considerations, the art has long soughta source other than petroleum for the massive quantities of rawmaterials that are needed to supply the demand for these light olefinmaterials.

The search for alternative materials for light olefin production has ledto the use of oxygenates such as alcohols and, more particularly, to theuse of methanol, ethanol, and higher alcohols or their derivatives suchas dimethyl ether, diethyl ether, etc., for example. Molecular sievessuch as microporous crystalline zeolite and non-zeolite catalysts,particularly silicoaluminophosphates (SAPO), are known to promote theconversion of oxygenates to hydrocarbon mixtures, particularlyhydrocarbon mixtures composed largely of light olefins.

Such processing of oxygenates to form light olefins is commonly referredto as a methanol-to-olefin (MTO) process, as methanol alone or togetherwith other oxygenate materials such as dimethyl ether (DME) is typicallyan oxygenate material most commonly employed therein. In practice, suchoxygenate conversion processing arrangements commonly produce ethyleneand propylene as main products and, as stand alone processing, canachieve propylene to ethylene product ratios up to about 1.4. Inaddition to the production of ethylene and propylene as main products,such processing also typically produces or results in smaller relativeamounts of highly olefinic C₄ and heavier hydrocarbon streams.

Commonly assigned U.S. Pat. No. 5,990,369 to Barger et al. discloses aprocess for the production of light olefins comprising olefins havingfrom 2 to 4 carbon atoms per molecule from oxygenate feedstock. Theprocess comprises passing the oxygenate feedstock to an oxygenateconversion zone containing a metal aluminophosphate catalyst to producea light olefin stream. The light olefin stream is fractionated and aportion of the products are metathesized to enhance the yield ofethylene, propylene and/or butylene products. Propylene can bemetathesized to produce an additional quantity of ethylene, or acombination of ethylene and butene can be metathesized to produce anadditional quantity of propylene. The combination of light olefinproduction and metathesis or disproportionation is disclosed asproviding flexibility such as to overcome the equilibrium limitations ofthe metal aluminophosphate catalyst in the oxygenate conversion zone. Inaddition, the invention thereof is disclosed as providing the advantageof extended catalyst life and greater catalyst stability in theoxygenate conversion zone.

While such processing can desirably result in the formation of increasedrelative amounts of propylene, further improvements such as to furtherenhance the relative amount of propylene production and recovery aredesired and have been sought.

SUMMARY OF THE INVENTION

A general object of the invention is to provide or result in improvedprocessing of an oxygenate-containing feedstock to light olefins.

A more specific objective of the invention is to overcome one or more ofthe problems described above.

The general object of the invention can be attained, at least in part,through a specified process for producing light olefins from anoxygenate-containing feedstock. In accordance with one embodiment, sucha process involves contacting the oxygenate-containing feedstock in anoxygenate conversion reactor with an oxygenate conversion catalyst andat reaction conditions effective to convert the oxygenate-containingfeedstock to form an oxygenate conversion effluent stream comprisinglight olefins and C₄+hydrocarbons, wherein the light olefins compriseethylene and the C₄+hydrocarbons comprise a quantity of butenes. Theoxygenate conversion stream is treated in a separation zone and forms afirst process stream comprising at least a portion of the ethylene fromthe oxygenate conversion effluent stream. At least a portion of theethylene from the first process stream is dimerized in a dimerizationzone to produce a dimerized stream comprising quantity of butenes. Atleast a portion of the butenes from the dimerized stream is contactedwith ethylene in a metathesis zone at effective conditions to produce ametathesis effluent stream comprising propylene, with at least a portionof this propylene desirably recovered therefrom. The process can furtherinclude forming in the separation zone a second process streamcomprising at least a portion of the quantity of butenes including aquantity of 1-butenes from the oxygenate conversion effluent stream andcontacting at least a portion of the quantity of butenes from the secondprocess stream with ethylene in the metathesis zone to producepropylene.

The prior art generally fails to provide processing schemes andarrangements for the conversion of an oxygenate-containing feedstock toolefins that maximizes production of propylene to as great an extent asmay be desired. Moreover, the prior art generally fails to provide aprocessing scheme and arrangement as effective and efficient as may bedesired in increasing the relative yield of propylene in associationwith the conversion of oxygenate materials to light olefins.

A process for producing light olefins from an oxygenate-containingfeedstock in accordance with another embodiment involves contacting anoxygenate-containing feedstock in an oxygenate conversion reactor withan oxygenate conversion catalyst and at reaction conditions effective toconvert the oxygenate-containing feedstock to form an oxygenateconversion effluent stream comprising light olefins and C₄+hydrocarbons,wherein the light olefins comprise a quantity of ethylene and theC₄+hydrocarbons comprises a quantity of diolefins and a quantity ofbutenes including a quantity of 1-butenes. The oxygenate conversioneffluent stream is separated in a separation zone and forms a firstprocess stream comprising at least a portion of the quantity of ethylenefrom the oxygenate conversion effluent stream. At least a portion of thequantity of ethylene from the first process stream is dimerized in adimerization zone to produce a dimerized stream comprising a residualquantity of ethylene, a quantity of butenes including a quantity of1-butenes, and a quantity of hexenes. At least a portion of the quantityof residual ethylene from the dimerized stream is metathesized with atleast a portion of the quantity of hexenes from the dimerized stream infirst metathesis zone to produce a first metathesis effluent streamcomprising a quantity of butenes including a quantity of 1-butenes and aquantity of propylene. At least a portion of the quantity of 1-butenesfrom the first metathesis effluent stream is isomerized in anisomerization zone to produce an isomerized stream comprising a quantityof 2-butenes. At least a portion of the 2-butenes from the isomerizedstream is metathesized with ethylene in a second metathesis zone toproduce a second metathesis effluent stream comprising propylene.Propylene can then be appropriately recovered from the second metathesiseffluent stream. The process can additionally include forming in theseparation zone a second process stream comprising at least a portion ofthe quantity of diolefins and at least a portion of the quantity ofbutenes, including a quantity of 1-butenes, from the oxygenateconversion effluent stream and metathesizing at least a portion of thequantity of butenes from the second process stream with ethylene in thesecond metathesis zone to produce propylene. The process can furtherinclude hydrogenating at least a portion of the diolefins from thesecond process stream in a hydrogenation zone to produce a hydrogenationeffluent stream comprising an additional quantity of 1-butenes andisomerizing at least a portion of the quantity of 1-butenes from thehydrogenation effluent stream in the isomerization zone to produce anadditional quantity of 2-butenes.

There is also provided a system for producing light olefins from anoxygenate-containing feedstock. In accordance with one embodiment, sucha system includes a reactor for contacting an oxygenate-containingfeedstream with an oxygenate conversion catalyst and converting theoxygenate-containing feedstream to an oxygenate conversion effluentstream comprising light olefins and C₄+hydrocarbons, wherein the lightolefins comprise a quantity of ethylene and the C₄+hydrocarbons comprisea quantity of butenes. A separation zone is provided for separating theoxygenate conversion effluent stream and forming a first process streamcomprising at least a portion of the quantity of ethylene from theoxygenate conversion effluent stream. A dimerization zone is providedfor dimerizing at least a portion of the quantity of ethylene from thefirst process stream to produce a dimerized stream comprising a quantityof butenes, including a quantity of 1-butenes, and a quantity ofhexenes. The system also includes a metathesis zone for contacting atleast a portion of the butenes from the dimerized stream with ethyleneto produce a metathesis effluent stream comprising propylene. A recoveryzone is also provided for recovering propylene from the metathesiseffluent stream.

As used herein, references to “light olefins” are to be understood togenerally refer to C₂ and C₃ olefins, i.e., ethylene and propylene,alone or in combination.

References to “C_(x) hydrocarbon” are to be understood to refer tohydrocarbon molecules having the number of carbon atoms represented bythe subscript “x”. Similarly, the term “C_(x)-containing stream” refersto a stream that contains C_(x) hydrocarbon. The term“C_(x)+hydrocarbons” refers to hydrocarbon molecules having the numberof carbon atoms represented by the subscript “x” or greater. Forexample, “C₄+hydrocarbons” include C₄, C₅ and higher carbon numberhydrocarbons. The term “C_(x)−hydrocarbons” refers to hydrocarbonmolecules having the number of carbon atoms represented by the subscript“x” or fewer. For example, “C₄−hydrocarbons” include C₄, C₃ and lowercarbon number hydrocarbons.

Other objects and advantages will be apparent to those skilled in theart from the following detailed description taken in conjunction withthe appended claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic process flow diagram illustrating aprocess for the conversion of oxygenates to olefins employing adimerization zone, to enhance the relative amount of butenes, and ametathesis zone, to enhance the relative yield of propylene, inaccordance with one embodiment.

FIG. 2 is a simplified schematic process flow diagram illustrating aprocess for the conversion of oxygenates to olefins employing adimerization zone, to enhance the relative amount of butenes, and ametathesis zone, to enhance the relative yield of propylene, inaccordance with another embodiment.

FIG. 3 is a simplified schematic process flow diagram illustrating aprocess for the conversion of oxygenates to olefins employing adimerization zone, to enhance the relative amount of butenes, anisomerization zone, to enhance the relative amount of 2-butenes, and ametathesis zone, to enhance the relative yield of propylene, inaccordance with a further embodiment.

FIG. 4 is a simplified schematic process flow diagram illustrating aprocess for the conversion of oxygenates to olefins employing adimerization zone, to enhance the relative amount of butenes, a firstmetathesis zone, to enhance the relative yield of propylene viametathesis of hexenes with ethylene, an isomerization zone, to enhancethe relative amount of 2-butenes, and a second metathesis zone, toenhance the relative yield of propylene via metathesis of 2-butenes withethylene, in accordance with an additional embodiment.

Those skilled in the art and guided by the teachings herein providedwill recognize and appreciate that the illustrated systems or processflow diagrams have been simplified by the elimination of various usualor customary pieces of process equipment including some heat exchangers,process control systems, pumps, fractionation systems, and the like. Itmay also be discerned that the process flow diagrams depicted in thefigures may be modified in many aspects without departing from the basicoverall concept of the invention.

DETAILED DESCRIPTION

Oxygenate-containing feedstocks can be converted to light olefins in acatalytic reactor and heavier hydrocarbons (e.g., C₄+hydrocarbons)formed during such processing can be subsequently treated such that atleast a portion of the quantity of ethylene formed upon such conversionis subsequently dimerized to form a stream containing at least butenes.Such butenes can then be metathesized to produce additional propylene.

As will be appreciated such processing may be embodied in a variety ofprocessing arrangements. As representative, FIG. 1 illustrates asimplified schematic process flow diagram for a processing scheme,generally designated with the reference numeral 10, for the conversionof oxygenates to olefins and employing a dimerization zone and ametathesis zone to enhance the yield of propylene, in accordance withone embodiment.

More particularly, an oxygenate-containing feedstock or feedstream insuch as generally composed of light oxygenates such as one or more ofmethanol, ethanol, dimethyl ether, diethyl ether, or combinationsthereof, is introduced via a line 12 into an oxygenate conversion zoneor reactor section 14 wherein the oxygenate-containing feedstockcontacts with an oxygenate conversion catalyst at reaction conditionseffective to convert the oxygenate-containing feedstock to form anoxygenate conversion effluent stream in a line 16 comprising fuel gashydrocarbons, light olefins, and C₄+hydrocarbons, in a manner as isknown in the art, such as, for example, utilizing a fluidized bedreactor.

As will be appreciated by those skilled in the art and guided by theteachings herein provided, such a feedstock may be commercial grademethanol, crude methanol or any methanol purity therebetween. Crudemethanol may be an unrefined product from a methanol synthesis unit.Those skilled in the art and guided by the teachings herein providedwill understand and appreciate that in the interest of factors such asimproved catalyst stability, embodiments utilizing higher puritymethanol feed may be preferred. Thus, suitable feeds in such embodimentsmay comprise methanol or a methanol and water blend, with possible suchfeeds having a methanol content of between about 65% and about 100% byweight, preferably a methanol content of between about 80% and about100% by weight and, in accordance certain embodiments, a methanolcontent of between about 95% and about 100% by weight.

A methanol-to-olefin unit feedstream may comprise between about 0 wt. %and about 35 wt. % and more preferably about 5 wt. % and about 30 wt. %water. The methanol in the feedstream may comprise between about 70 wt.% and about 100 wt. % and more preferably between about 75 wt. % andabout 95 wt. % of the feedstream. The ethanol in the feedstream maycomprise between about 0.01 wt. % and about 0.5 wt. % and more typicallybetween about 0.1 wt. % and about 0.2 wt. % of the feedstream althoughhigher concentrations may be beneficial. When methanol is the primarycomponent in the feedstream, the higher alcohols in the feedstream maycomprise between about 200 wppm and about 2000 wppm and more typicallyabout between about 500 wppm and 1500 wppm. Additionally, when methanolis the primary component in the feedstream, dimethyl ether may comprisebetween about 100 wppm and about 20,000 wppm and more typically betweenabout 200 wppm and about 10000 wppm.

The invention, however, also contemplates and encompasses embodimentswherein the oxygenate-containing feedstock includes dimethyl ether,either alone or in combination with water, methanol or in combinationwith both water and methanol, for example. The invention specificallyencompasses embodiments wherein the oxygenate-containing feedstock isessentially dimethyl ether either alone or with no more thaninsubstantial amounts of other oxygenate materials.

Reaction conditions for the conversion of oxygenate to light olefins areknown to those skilled in the art. Preferably, in accordance withparticular embodiments, reaction conditions comprise a temperaturebetween about 200° C. and about 700° C., more preferably between about300° C. and about 600° C., and most preferably between about 400° C. andabout 550° C. In addition, reactor operating pressures typically arepreferably superatmospheric and such as generally range from about 69kPa gauge to about 689 kPa gauge (about 10 psig to about 100 psig), asmay be required to accommodate sufficient pressure at the compressorsection.

As will be appreciated by those skilled in the art and guided by theteachings herein provided, the reaction conditions are generallyvariable such as dependent on the desired products. For example, ifincreased ethylene production is desired, then operation at a reactortemperature between about 475° C. and about 550° C. and more preferablybetween about 500° C. and about 520° C., may be preferred. If increasedpropylene production is desired, then operation at a reactor temperaturebetween about 350° C. and about 475° C. and more preferably betweenabout 400° C. and about 430° C. may be preferred. In addition, higherpressures tend to yield slightly more propylene relative to ethylene.

The light olefins produced can have a ratio of ethylene to propylene ofbetween about 0.5 and about 2.0 and preferably between about 0.75 toabout 1.25. If a higher ratio of ethylene to propylene is desired, thenthe reaction temperature is generally desirably higher than if a lowerratio of ethylene to propylene is desired. In accordance with oneembodiment, a feed temperature of between about 120° C. and about 210°C. is preferred. In accordance with another embodiment, a feedtemperature of between about 180° C. and about 210° C. is preferred. Inaccordance with a further embodiment, the temperature is desirablymaintained below 210° C. to avoid or minimize thermal decomposition.

The oxygenate conversion zone 14 produces or results in the formation ofan oxygenate conversion effluent stream in the line 16 such as generallycomprising fuel gas hydrocarbons, light olefins, and C₄+hydrocarbons.The light olefins comprise a quantity of ethylene and theC₄+hydrocarbons generally typically comprise a quantity of diolefins aswell as a quantity of butenes including a quantity of 1-butenes.

The oxygenate conversion effluent stream, or at least a portion thereof,is passed via the line 16 to an oxygenate conversion effluent streamtreatment or separation of zone, generally designated by referencenumeral 18, wherein the oxygenate conversion effluent stream isresolved, e.g., fractionated, using conventional separation means, toform a first process stream in a line 20 comprising at least a portionof the quantity of ethylene from the oxygenate conversion effluentstream. Such conventional separation means are described in greaterdetail below in conjunction with, for example, FIG. 2.

The oxygenate conversion effluent stream in the line 16 may be furtherresolved, e.g., fractionated, using conventional separation means, toform a second process stream in a line 22 comprising at least a portionof the quantity of diolefins and at least a portion of the quantity ofbutenes, including a portion of the quantity of 1-butenes, from theoxygenate conversion effluent stream. Such conventional separation meansare described in greater detail below in conjunction with, for example,FIG. 2. Other process streams that may be separated from the oxygenateconversion effluent stream the line 16 in the treatment or separationzone 18 include, for example, a propylene product stream 24, a paraffinsstream 26 comprising, for example, propane, and a stream 28 of heavyhydrocarbons generally typically comprising C₅+hydrocarbons.

The first process stream, or at least a portion thereof, is passed vialines 20, 23 and 50 to a dimerization zone 30 wherein at least a portionof the quantity of ethylene from the first product stream is dimerizedover a dimerization catalyst and at reaction conditions effective toresult in or produce a dimerized stream in a line 32 comprising aquantity of butenes. In accordance with certain embodiments, a dragstream 21 may be provided to reduce the build-up of selected undesirablehydrocarbon components such as, for example, ethane, in the dimerizationzone 30.

The dimerization reaction can generally be carried out under conditionsand employ catalysts such as are known in the art. For example, suchdimerization catalysts may be homogeneous or heterogeneous, with theheterogeneous catalysts typically being preferred. The dimerizationcatalyst may preferably comprise a catalytically effective amount of atransition metal component. Preferred transition metals for use in thepractice of the present invention include tungsten, molybdenum, nickel,rhenium, and combinations thereof, with nickel being preferred. Thetransition metal component may be present as elemental metal and/or oneor more compounds of the metal. If the catalyst is heterogeneous, it ispreferred that the transition metal component be associated with asupport. Any suitable support material may be employed provided that itdoes not substantially interfere with the feedstock components.Preferably, the support material comprises silica, silica-alumina,Y-zeolite, X-zeolite, a polymeric material, sulfated alumina, and ZSM-5.Silica-alumina is a particularly preferred support material. If asupport material is employed, the amount of transition metal componentused in combination with the support material may vary widely,depending, for example, on the particular application involved and/orthe transition metal being used.

Typical or usual dimerization reaction conditions, such as whenemploying a nickel on silica-alumina catalyst, may involve a temperatureof about 80° C. to about 120° C. and typically a pressure of about 3 MPa(about 435 psia). Lower temperatures are generally, typically preferredto promote dimerization of ethylene to butenes and pentenes and to limitor prevent skeletal isomerization of the butenes to other olefinproducts.

Desirably, the extent of the dimerization reaction can be controlledsuch as by controlling the temperature and/or pressure within thedimerization zone 30 and/or by controlling the amount of ethylene fromthe first process stream in line 20 which bypasses the dimerization zone30 (i.e., via the drag stream 21) so as to control the ethylene tobutene ratio in the dimerized stream in the line 32. In accordance withone embodiment, operating conditions in the dimerization zone 30 arecontrolled to maintain an ethylene to butene ratio of about 1:1 to about5:1 and a butene selectivity in the dimerized stream of at least about80%, and preferably at least about 90%.

The dimerized stream, or at least a portion thereof, is introduced viathe line 32 into a metathesis zone 34 and under effective conditionswherein at least a portion of the butenes from the dimerized stream iscontacted with ethylene to produce a metathesis effluent stream in aline 36 comprising propylene. In accordance with certain embodiments,the metathesis effluent stream can additionally include a residualquantity of ethylene, a residual quantity of butenes, a quantity of C₅and/or C₆ hydrocarbons such as generally composed of pentenes andhexenes, and a quantity of heavy hydrocarbon materials generallycomposed of materials heavier than hexane.

The metathesis reaction can generally be carried out under conditionsand employ catalysts such as are known in the art. In accordance withone embodiment, a metathesis catalyst such as containing a catalyticamount of at least one of molybdenum oxide and tungsten oxide issuitable for the metathesis reaction. Conditions for the metathesisreaction in the vapor phase generally include a reaction temperatureranging from about 20° C. to about 450° C., preferably 250° C. to 350°C., and pressures varying from about atmospheric to upwards of 20.6 MPagauge (3000 psig), preferably between about 3000 kPa gauge to 3500 kPagauge (435 psig to 510 psig), although higher pressures can be employedif desired. Conditions for the metathesis reaction in the liquid phasegenerally include a reaction temperature ranging from about 25° C. to50° C. and pressures sufficient to maintain a liquid phase. Catalystswhich are active for the metathesis of olefins and which can be used inthe process of this invention are of a generally known type. In thisregard, reference is made to “Journal of Catalysis”, 13 (1969) pages99-114, to “Applied Catalysis”, 10 (1984) pages 29-229 and to “CatalysisReview”, 3 (1) (1969) pages 37-60.

Such metathesis catalysts may be homogeneous or heterogeneous, with theheterogeneous catalysts being preferred. The metathesis catalystpreferably comprises a catalytically effective amount of transitionmetal component. Preferred transition metals for use in the presentinvention include tungsten, molybdenum, nickel, rhenium, andcombinations thereof. The transition metal component may be present aselemental metal and/or one or more compounds of the metal. If thecatalyst is heterogeneous, it is generally preferred that the transitionmetal component be associated with a support. Any suitable supportmaterial may be employed provided that it does not substantiallyinterfere with the feedstock components or the lower olefin componentconversion. Preferably, the support material is an oxide, such assilica, alumina, titania, zirconia, and combinations thereof. Silica isa particularly preferred support material. If a support material isemployed, the amount of transition metal component used in combinationwith the support material may vary widely, depending, for example, onthe particular application involved and/or the transition metal beingused. Preferably, the transition metal comprises about 1% to about 20%,by weight (calculated as elemental metal) of the total catalyst. Themetathesis catalyst advantageously comprises a catalytically effectiveamount of at least one of the above-noted transition metals, and iscapable of promoting olefin metathesis. The catalyst may also contain atleast one activating agent present in an amount to improve theeffectiveness of the catalyst. Various activating agents may beemployed, including activating agents which are well known in the art tofacilitate metathesis reactions. Light olefins metathesis catalysts can,for example, desirably be complexes of tungsten (W), molybdenum (Mo), orrhenium (Re) in a heterogeneous or homogeneous phase.

The metathesis effluent stream, or at least a portion thereof, isintroduced via the line 36 into a metathesis fractionation zone 38wherein the metathesis effluent stream is resolved, e.g., fractionated,by conventional separation means into a propylene product stream 40 anda butenes fraction in a line 42 generally composed of at least a portionof the residual quantity of butenes from the metathesis effluent stream.The butenes fraction (i.e., a butenes recycle stream), or at least aportion thereof, can be recycled back into the processing scheme 10 suchas, for example, by introducing the butenes fraction in the line 42 intothe metathesis zone 34 via lines 43 and 44. In embodiments wherein suchbutenes fraction from the metathesis fractionation zone 38 is recycledto the metathesis zone 34, a drag or purge stream 46 may be provided toreduce the build-up of selected hydrocarbon components such as, forexample, isobutene, in the process loop.

The metathesis effluent stream in the line 36, or at least a portionthereof, can be further treated in the metathesis fractionation zone 38to produce or result in the formation of an ethylene recycle stream in aline 48 generally composed of at least a portion of the residualquantity of ethylene from the metathesis effluent stream in the line 36and which can be subsequently recycled to the dimerization zone 30. Inpractice, at least a portion of the ethylene recycle stream in the line48 can be combined with the first process stream in the line 20 and suchcombined stream can be introduced into the dimerization zone 30 via theline 50.

The metathesis effluent stream in the line 36 can be further resolved,e.g., fractionated, in the metathesis fractionation zone 38, such as byconventional distillation methods, to produce a third stream generallycomposed of at least a portion of the C₅ and/or C₆ hydrocarbons whichcan be introduced into the oxygenate conversion effluent treatment zone18 via a line 52 for further processing. Alternatively, the thirdprocess stream can be removed from the processing scheme 10. Forexample, the third process stream, or a portion thereof, can be used asfuel.

The metathesis effluent stream in the line 36 can be further resolved,e.g., fractionated, in the metathesis fractionation zone 38, such as byconventional distillation methods, to produce a heavy hydrocarbon purgestream 54. In practice, the heavy hydrocarbon purge stream 54, or aportion thereof, can be used as fuel. For example, for locations inproximity to refineries, such materials or select portions thereof canbe blended into a gasoline pool. Additionally or alternatively,depending upon the specifications as to the olefin content in a feed toa synthesis gas unit, the heavy hydrocarbon purge stream 54 or a portionthereof can be recycled to a front-end synthesis gas unit.

In accordance with certain embodiments, at least a portion of thebutenes including a portion of the quantity of 1-butenes from the secondprocess stream in the line 22 can be contacted with ethylene in themetathesis zone 34 to produce an additional quantity of propylene.Additionally or alternatively, the processing scheme 10 can include ahydrogenation zone 56 wherein at least a portion of the quantity ofdiolefins from the second process stream in the line 22 can behydrogenated to produce a hydrogenation effluent stream in a line 58comprising an additional quantity of 1-butenes. In practice, at least aportion of the hydrogenation effluent stream in the line 58, alone or incombination with at least a portion of the butenes recycle stream in theline 42, can be introduced into the metathesis zone 34 via the line 44to produce an additional quantity of propylene.

In accordance with another embodiment, as illustrated in FIG. 2, aprocessing scheme 100 for producing light olefins from anoxygenate-containing feedstock involves introducing, via lines 102 and176, an oxygenate-containing feedstock or feedstream such as generallycomposed of light oxygenates such as one or more of methanol, ethanol,dimethyl ether, diethyl ether, or combinations thereof, into anoxygenate conversion zone or reactor section 104 wherein theoxygenate-containing feedstock contacts with an oxygenate conversioncatalyst at reaction conditions effective to convert theoxygenate-containing feedstock to form an oxygenate conversion effluentstream in a line 106 comprising fuel gas hydrocarbons, light olefinsincluding ethylene, and C₄+hydrocarbons including butenes and diolefins(e.g., butadienes), in a manner as is known in the art, such as, forexample, utilizing a fluidized bed reactor.

The oxygenate conversion effluent stream in the line 106 can be furtherprocessed in a separation or treatment zone 108 wherein the oxygenateconversion effluent stream, or at least a portion thereof, may beseparated or fractionated, such as by conventional distillation methods,to provide one or more process streams.

In accordance with certain embodiments, the oxygenate conversioneffluent stream, or a select portion thereof, is passed via lines 106and 164 to a deethanizer zone 110. In the deethanizer zone 110, theoxygenate conversion effluent stream is fractionated, such as byconventional distillation methods, such as to provide or form adeethanizer overhead stream in a line 112 comprising C₂−hydrocarbonsincluding methane, ethane, ethylene, acetylene and inert species such asN₂, CO, and the like, and a deethanized C₃+bottoms stream in a line 114comprising components heavier than ethane, such as propylene, propane,mixed butenes, diolefins (e.g., butadienes) and/or butane.

The deethanizer overhead stream, or at least a portion thereof, ispassed via the line 112 to an acetylene saturation zone 116, wherein atleast a portion of the acetylene from the deethanizer overhead stream istreated to produce a treated stream in a line 120 comprising anadditional quantity of ethylene.

A demethanizer zone can generally typically be employed to avoid theundesired build-up of carbon dioxide in the processing scheme 100 and toprolong the life of dimerization and/or metathesis catalysts employedtherein. In accordance with one embodiment, the treated stream, or atleast a portion thereof, is passed via the line 120 to a demethanizer orC₂ fractionation zone 122 wherein the treated stream is fractionated,such as by conventional distillation methods, to form a demethanizeroverhead stream in a line 124 comprising methane and inert species suchas N₂, CO, and the like, if present, and a first process stream in aline 126 comprising C₂ materials including ethane and ethylene.

The first process stream, or at least a portion thereof, is passed vialines 126, 127 and 129 to a dimerization zone 128 wherein at least aportion of the ethylene from the first process stream is dimerized overa dimerization catalyst and at reaction conditions effective to resultin or produce a dimerized stream in a line 130 comprising a quantity ofbutenes, a quantity of hexenes and a residual quantity of ethylene. Thedimerization reaction can generally employ catalysts and can generallybe carried out as described in detail above in conjunction with thedimerization zone 30, as illustrated in FIG. 1.

The dimerized stream, or at least a portion thereof, is passed via theline 130 to a metathesis zone 132 wherein at least a portion of thequantity of butenes and/or at least a portion of the quantity of hexenesfrom the dimerized stream is contacted with a quantity of ethylene overa metathesis catalyst to produce a metathesis effluent stream in a line136 comprising propylene. The metathesis reaction can generally employcatalysts and be carried out as described in detail above in conjunctionwith the metathesis zone 34, as illustrated in FIG. 1.

In accordance with certain embodiments, at least a portion of theresidual quantity of ethylene from the dimerized stream in the line 130can provide a quantity of ethylene to support the metathesis reaction.In accordance with certain other embodiments, a portion of the firstprocess stream in the line 126 can be introduced into the metathesiszone 132 via a line 134 to provide the quantity of ethylene for themetathesis reaction

In accordance with certain embodiments, an additional quantity ofbutenes can be introduced into the metathesis zone 132 to produce anadditional quantity of propylene. In practice, such additional quantityof butenes may be derived or formed, for example, from of thedeethanized C₃+bottoms stream in the line 114.

The deethanized C₃+bottoms stream, or at least a portion thereof, can bepassed via the line 114 to a depropanizer zone 142. In the depropanizerzone 142, the deethanized C₃+bottoms stream in the line 114 isfractionated, such as by conventional distillation methods, to form adepropanizer overhead stream in a line 144 comprising C₃ materialsincluding propylene and propane and a depropanized stream in a line 146comprising C₄+hydrocarbons comprising components heavier than propane,including a quantity of mixed butenes and a quantity of diolefins. Thedepropanizer overhead stream, or at least a portion thereof, can bepassed via the line 144 to a C₃ splitter 148, wherein the depropanizeroverhead stream is treated, e.g., fractionated, such as by conventionaldistillation methods, to provide an overhead propylene product stream150 such as generally composed of propylene and a bottoms stream 152such as generally composed of propane.

The depropanized stream, or at least a portion thereof, can be passedvia the line 146 to a debutanizer or C₄ fractionation zone 154. In thedebutanizer zone 154, the depropanized stream can be treated, e.g.,fractionated, such as by conventional distillation methods, to formdebutanizer overhead stream in a line 156 comprising C₄ materialsincluding a quantity of mixed butenes and a quantity of diolefins (i.e.,butadienes) and debutanized stream in a line 158 generally composed ofmaterials heavier than butane.

In accordance with certain embodiments, at least a portion of thequantity of mixed butenes from the debutanizer overhead stream in theline 156 can be introduced directly into the metathesis zone 132. Inaccordance with certain other embodiments, the debutanizer overheadstream, or at least a portion thereof, is passed via lines 156 and 157to a hydrogenation zone 160 wherein at least a portion of the quantityof diolefins from the debutanizer overhead stream are selectivelyhydrogenated to produce a hydrogenation effluent stream in a line 162comprising an additional quantity of butenes. The additional quantity ofbutenes from the hydrogenation effluent stream, or at least a portionthereof, can be subsequently introduced into the metathesis zone 132 viathe line 162 to produce an additional quantity of propylene viametathesis with ethylene.

Propylene is desirably recovered from the metathesis effluent stream inthe line 136. In accordance with one embodiment, propylene is recoveredby introducing the metathesis effluent stream, or a select portionthereof, via the line 136 into the separation zone 108. For example, themetathesis effluent stream in the line 136, or at least a portionthereof, can be combined with the oxygenate conversion effluent streamin the line 106 and such combined stream can be introduced into theseparation section 108 via a line 164 wherein propylene is recoveredfrom such combined stream according to the process described above inconjunction with the deethanizer zone 110, the depropanizer zone 142 andthe C₃ splitter 148.

Alternatively, the metathesis effluent stream, or at least a portionthereof, can be passed via the line 136 to a metathesis fractionationzone (not shown) wherein the metathesis effluent stream is resolved,e.g., fractionated, by conventional separation means into a propyleneproduct stream and a higher hydrocarbon fraction including butene whichcan be recycled back into the processing scheme 100 such as, forexample, back into the any one of the deethanizer zone 110, thedepropanizer zone 142, the debutanizer zone 154, or the metathesis zone132. In embodiments wherein such higher hydrocarbon fraction from themetathesis fractionation zone is recycled to the metathesis zone 132, adrag stream can be provided to reduce the build-up of selected higherhydrocarbon components such as, for example, isobutene, in the processloop.

In accordance with certain embodiments, a drag stream 166 can beprovided to reduce build-up of ethane in the processing scheme 100. Theethane-containing drag stream 166 can, for example, be disposed betweenthe demethanizer zone 126 and the dimerization zone 128, i.e., drawn offfrom the first process stream in the line 126. The ethane-containingdrag stream 166, or a portion thereof, can be recycled to a front-endsynthesis gas unit or, if such unit is not readily available, can beused as fuel.

In accordance with certain embodiments, the processing scheme 100 canadditionally include a C₄ purge stream 168 to avoid undesired build-upnonreacting materials (e.g., saturates) and, particularly, isobutenes,that might otherwise accumulate in the process loop. The C₄ purge stream168 can be disposed between the debutanizer zone 154 and the metathesiszone 132, i.e., drawn off from the debutanizer overhead stream in theline 156.

In accordance with certain embodiments, the debutanized stream in theline 158, or at least a portion thereof, can be further treated, e.g.,fractionated, such as by conventional distillation methods, in a heavyhydrocarbon separation zone 170. In the heavy hydrocarbon separationzone 170, the debutanized bottoms stream is treated to form an overheadstream in a line 172 generally composed of C₅ and/or C₆ hydrocarbons anda heavy hydrocarbon bottom stream 174 generally comprising componentsheavier than hexane. In practice the overhead stream, or a portionthereof, can be directly recycled to the oxygenate conversion zone 104for further processing. Alternatively, at least a portion of theoverhead stream in the line 172 can be combined with theoxygenate-containing feedstock and such combined stream can beintroduced into the oxygenate conversion zone 104 via a line 176. Inpractice, the heavy hydrocarbon bottom stream 174, or a portion thereof,can be used as fuel. For example, for locations in proximity torefineries, such materials or select portions thereof can be blendedinto a gasoline pool. Additionally or alternatively, depending upon thespecifications as to the olefin content in a feed to a synthesis gasunit, the heavy hydrocarbon bottoms stream 174, or a portion thereof,can be recycled to a front-end synthesis gas unit.

In accordance with an additional embodiment, as illustrated in FIG. 3, aprocessing scheme 200 for producing light olefins from anoxygenate-containing feedstock involves introducing, via lines 202 and276, an oxygenate-containing feedstock or feedstream such as generallycomposed of light oxygenates such as one or more of methanol, ethanol,dimethyl ether, diethyl ether, or combinations thereof, into anoxygenate conversion zone or reactor section 204 wherein theoxygenate-containing feedstock contacts with an oxygenate conversioncatalyst at reaction conditions effective to convert theoxygenate-containing feedstock to form an oxygenate conversion effluentstream in a line 206 comprising fuel gas hydrocarbons, light olefinsincluding ethylene, and C₄+hydrocarbons including butenes and diolefins,in a manner as is known in the art, such as, for example, utilizing afluidized bed reactor.

The oxygenate conversion effluent stream in the line 206 can be furtherprocessed in a separation or treatment zone 208 wherein the oxygenateconversion effluent stream, or at least a portion thereof, may beseparated or fractionated, such as by conventional distillation methods,to provide one or more process streams.

In accordance with certain embodiments, the oxygenate conversioneffluent stream, or a select portion thereof, is passed via lines 206and 264 to a demethanizer zone 210. In the demethanizer zone 210, theoxygenate conversion effluent stream is fractionated, such as byconventional distillation methods, such as to provide or form ademethanizer overhead stream in a line 212 comprising methane and inertspecies such as N₂, CO, and the like, if present, and a demethanizedC₂+bottoms stream in a line 214 comprising components heavier thanmethane, such as ethylene, ethane, propylene, propane, mixed butenes,diolefins (e.g., butadienes) and/or butane. The demethanizer overheadstream in the line 212, or a portion thereof, can be used as fuel.

The demethanized C₂+bottoms stream, or a select portion thereof, ispassed via the line 214 to a deethanizer zone 216. In the deethanizerzone 216, the demethanized C₂+bottoms stream is fractionated, such as byconventional distillation methods, such as to provide or form adeethanizer overhead stream in a line 218 comprising C₂ materialsincluding ethane, ethylene, and possibly also some acetylene, and adeethanized C₃+bottoms stream in a line 220 comprising componentsheavier than ethane, such as propylene, propane, mixed butenes,diolefins (e.g., butadienes) and/or butane.

The deethanizer overhead stream, or at least a portion thereof, ispassed via the line 218 to an acetylene saturation zone 222, wherein atleast a portion of the acetylene from the deethanizer overhead stream istreated to produce a first process stream in a line 224 comprising anadditional quantity of ethylene.

The first process stream, or at least a portion thereof, is passed vialines 224, 225 and 227 to a dimerization zone 226 wherein at least aportion of the ethylene from the first process stream is dimerized overa dimerization catalyst and at reaction conditions effective to resultin or produce a dimerized stream in a line 228 comprising a quantity ofbutenes, a quantity of hexenes and a residual quantity of ethylene. Thedimerization reaction can generally employ catalysts and be carried outas described in detail above in conjunction with the dimerization zone30, as illustrated in FIG. 1.

The dimerized stream, or at least a portion thereof, is passed via theline 228 to a metathesis zone 230 wherein at least a portion of thequantity of butenes and/or at least a portion of the quantity of hexenesfrom the dimerized stream is contacted with a quantity of ethylene overa metathesis catalyst to produce a metathesis effluent stream in a line232 comprising propylene. The metathesis reaction can generally employcatalysts and be carried out as described in detail above in conjunctionwith the metathesis zone 34, as illustrated in FIG. 1.

In accordance with certain embodiments, at least a portion of theresidual quantity of ethylene from the dimerized stream in the line 228can provide the quantity of ethylene to support the metathesis reaction.In accordance with certain embodiments, a portion of the first processstream in the line 224 can be introduced into the metathesis zone 230via a line 236 to provide the quantity of ethylene for the metathesisreaction

In accordance with certain embodiments, an additional quantity ofbutenes can be introduced into the metathesis zone 230 to produce anadditional quantity of propylene. In practice, such additional quantityof butenes may be derived or formed, for example, from of thedeethanized C₃+bottoms stream in the line 220.

The deethanized C₃+bottoms stream, or at least a portion thereof, can bepassed via the line 220 to a depropanizer zone 238. In the depropanizerzone 238, the deethanized C₃+bottoms stream is fractionated, such as byconventional distillation methods, to form a depropanizer overheadstream in a line 240 comprising C₃ materials including propylene andpropane and a depropanized stream in a line 242 comprisingC₄+hydrocarbons comprising components heavier than propane, including aquantity of mixed butenes and a quantity of diolefins. The depropanizeroverhead stream, or at least a portion thereof, can be passed via theline 240 to a C₃ splitter 244, wherein the depropanizer overhead streamis treated, e.g., fractionated, such as by conventional distillationmethods, to provide an overhead propylene product stream 246 such asgenerally composed of propylene and a bottoms stream 248 such asgenerally composed of propane.

The depropanized stream, or at least a portion thereof, can be passedvia the line 242 to a debutanizer zone 250. In the debutanizer zone 250,the depropanized stream can be treated, e.g., fractionated, such as byconventional distillation methods, to form debutanizer overhead streamin a line 252 comprising C₄ materials including a quantity of 1-butenes,a quantity of 2-butenes, a quantity of isobutenes, and a quantity ofdiolefins (e.g., butadienes) and a debutanized stream in a line 254generally composed of materials heavier than butane.

The debutanizer overhead stream, or at least a portion thereof, ispassed via lines 252 and 253 to a hydrogenation zone 256 wherein atleast a portion of the quantity of diolefins from the debutanizeroverhead stream are selectively hydrogenated to produce a hydrogenationeffluent stream in a line 258 comprising an additional quantity ofbutenes including a quantity of 1-butenes.

It has been found that the metathesis reaction of butenes with ethyleneover a metathesis catalyst to produce propylene is favored where thebutenes are in the form of 2-butenes rather than 1-butenes. Thus, inaccordance with one embodiment, and as described in greater detailbelow, the hydrogenation effluent stream, or at least a portion thereof,is passed via the line 258 to an isomerization zone 260 for isomerizingat least a portion of the quantity of 1-butenes therein contained toform an isomerized stream in a line 262 comprising an increased quantityof 2-butenes.

As will be appreciated, such isomerization of 1-butenes to 2-butenes candesirably occur over a suitable isomerization catalyst at selectedappropriate isomerization reaction conditions. In accordance withcertain embodiments, the 1-butene to 2-butene isomerization reaction canbe a hydroisomerization as it is generally conducted in the presence ofa hydrogen atmosphere to facilitate the double bond migration, but suchthat the use of hydrogen is minimized to avoid undesirable hydrogenationside reactions. The catalysts typically employed in such processing arecommonly based on noble metals (palladium, rhodium, platinum, etc.)deposited on an inert alumina support; palladium is normally preferred.Typical or usual reaction conditions may involve a temperature of about100° C. to about 150° C. and typically a pressure of about 1.5 MPa to 2MPa (about 215 psia to 300 psia). The feed to the hydroisomerizationreactor is usually preheated by exchange with the reactor effluent andby steam. Such a heated feed then enters the reactor, which typicallyoperates in a mixed phase with one or more catalyst beds. After coolingthe isomerization products are typically flashed to remove excesshydrogen gas. The reaction temperature is generally chosen so as tomaximize conversion to 2-butene (favored by lower temperatures) whilestill having a reasonable rate of reaction; hence it is commonlydesirable to operate at a temperature of less than 150° C.

In accordance with certain other embodiments, the 1-butene to 2-buteneisomerization reaction can be carried out in the absence of hydrogen.For example, the isomerization reaction can be carried out in thepresence of a catalyst comprising ruthenium oxide and an alkali metaloxide base on an alumina or silica support. Typical or usual reactionconditions may involve a temperature of about 100° C. to about 200° C.and typically a pressure of up to about 6.9 MPa (about 1000 psig). Whena flow of butenes is fed continuously over the catalyst, weight hourlyspace velocity (WHSV) can be in a range of from about 0.2 to about 10,and, typically, about 2 to about 4.

Desirably, the isomerized stream will contain 2-butene and 1-butene in amolar ratio of at least 8, e.g., at least 8 moles of 2-butenes per moleof 1-butenes, and, in accordance with at least certain embodiments, amolar ratio of greater than 10, e.g., more than 10 moles of 2-butene permole of 1-butene. If fractionated, the residual 1-butene can be recycledto the isomerization reactor.

The isomerized stream, or at least a portion thereof, can be introducedvia the line 262 into the metathesis zone 230 wherein the 2-butenes canbe metathesized with ethylene to produce an additional quantity ofpropylene.

Propylene is desirably recovered from the metathesis effluent stream inthe line 232. In accordance with one embodiment, propylene is recoveredby introducing the metathesis effluent stream, or a select portionthereof, into the separation zone 208. For example, the metathesiseffluent stream in the line 232, or at least a portion thereof, can becombined with the oxygenate conversion effluent stream in the line 206and such combined stream can be introduced into the separation section208 via a line 264 wherein propylene is recovered from such combinedstream according to the process described above in conjunction with thedemethanizer zone 210, the deethanizer zone 216, the depropanizer zone238 and the C₃ splitter 244.

Alternatively, the metathesis effluent stream in the line 232, or atleast a portion thereof, can be passed to a metathesis fractionationzone (not shown) wherein the metathesis effluent stream is resolved,e.g., fractionated, by conventional separation means into a propyleneproduct stream and a higher hydrocarbon fraction including butene whichcan be recycled back into the processing scheme such as, for example,back into the any one of the demethanizer zone 210, the deethanizer zone216, the depropanizer zone 238, the debutanizer zone 250, or themetathesis zone 230. In embodiments wherein such higher hydrocarbonfraction from the metathesis fractionation zone is recycled to themetathesis zone 230, a drag stream can be provided to reduce thebuild-up of selected higher hydrocarbon components such as, for example,isobutene, in the process loop.

In accordance with certain embodiments, a drag stream 266 can beprovided to reduce build-up of ethane in the processing scheme 200. Theethane-containing drag stream 266 can, for example, be disposed betweenthe acetylene saturation zone 222 and the dimerization zone 226, i.e.,drawn off from the first process stream in the line 224. Theethane-containing drag stream 266, or a portion thereof, can be recycledto a front-end synthesis gas unit or, if such unit is not readilyavailable, can be used as fuel.

In accordance with certain embodiments, the processing scheme 200 canadditionally include a C₄ purge stream 268 to avoid undesired build-upnonreacting materials (e.g., saturates) and, particularly, isobutenesthat might otherwise accumulate in the process loop. The C₄ purge stream268 can be disposed between the debutanizer zone 250 and thehydrogenation zone 256, i.e., drawn off from the debutanizer overheadstream in the line 252.

In accordance with certain embodiments, the debutanized stream in theline 254, or at least a portion thereof, can be further treated, e.g.,fractionated, such as by conventional distillation methods, in a heavyhydrocarbon separation zone 270. In the heavy hydrocarbon separationzone 270, the debutanized bottoms stream is treated to form an overheadstream in a line 272 generally composed of C₅ and/or C₆ hydrocarbons anda heavy hydrocarbon bottom stream 274 generally comprising componentsheavier than hexane. In practice the overhead stream, or a portionthereof, can be directly recycled to the oxygenate conversion zone 204for further processing. Alternatively, at least apportion of theoverhead stream in the line 272 can be combined with theoxygenate-containing feedstock in the line 202 and such combined streamcan be introduced into the oxygenate conversion zone 204 via a line 276.In practice, the heavy hydrocarbon bottom stream 274, or a portionthereof, can be used as fuel. For example, for locations in proximity torefineries, such materials or select portions thereof can be blendedinto a gasoline pool. Additionally or alternatively, depending upon thespecifications as to the olefin content in a feed to a synthesis gasunit, the heavy hydrocarbon bottoms stream 274, or a portion thereof,can be recycled to a front-end synthesis gas unit.

In accordance with a further embodiment, as illustrated in FIG. 4, aprocessing scheme 300 for producing olefins from an oxygenate-containingfeedstock involves introducing, via a line 302, an oxygenate-containingfeedstock or feedstream such as generally composed of light oxygenatessuch as one or more of methanol, ethanol, dimethyl ether, diethyl ether,or combinations thereof, into an oxygenate conversion zone or reactorsection 304 wherein the oxygenate-containing feedstock contacts with anoxygenate conversion catalyst at reaction conditions effective toconvert the oxygenate-containing feedstock to form an oxygenateconversion effluent stream in a line 306 comprising fuel gashydrocarbons, light olefins, and C₄+hydrocarbons, in a manner as isknown in the art, such as, for example, utilizing a fluidized bedreactor. The light olefins comprise a quantity of ethylene and theC₄+hydrocarbons generally typically comprise a quantity of diolefins aswell as a quantity of butenes including a quantity of 1-butenes.

The oxygenate conversion effluent stream, or at least a portion thereof,is passed via the line 306 to an oxygenate conversion effluent streamtreatment or separation zone 308, wherein the oxygenate conversioneffluent stream is resolved, e.g., fractionated, by conventionalseparation means to form a first process stream in a line 310 comprisingat least a portion of the quantity of ethylene from the oxygenateconversion effluent stream. Such conventional separation means aredescribed in greater detail above in conjunction with, for example, FIG.2 and FIG. 3.

The oxygenate conversion effluent stream in the line 306, or at least aportion thereof, may be further resolved, e.g., fractionated, byconventional separation means to form a second process stream in a line312 comprising at least a portion of the quantity of diolefins and atleast a portion of the quantity of butenes, including a portion of thequantity of 1-butenes, from the oxygenate conversion effluent stream.Other process streams that may be separated from the oxygenateconversion effluent stream in the line 306 in the treatment orseparation zone 308 include, for example, a propylene product stream314, a paraffins stream 316 comprising, for example, propane, and aheavy hydrocarbon stream 318 generally typically comprisingC₅+hydrocarbons.

The first process stream, or at least a portion thereof, is passed vialines 310, 311 and 313 to a dimerization zone 320 wherein at least aportion of the quantity of ethylene from the first product stream isdimerized over a dimerization catalyst and at reaction conditionseffective to result in or produce a dimerized stream in a line 322comprising a residual quantity of ethylene, a quantity of butenes,including a quantity of 1-butenes, and a quantity of hexenes. Thedimerization reaction can generally employ catalysts and can generallybe carried out as described in detail above in conjunction with thedimerization zone 30, as illustrated in FIG. 1.

In accordance with certain embodiments, a drag stream 324 may beprovided to reduce the build-up of selected hydrocarbon components suchas, for example, ethane in the dimerization zone 320.

The dimerized stream, or at least a portion thereof, is passed via theline 322 to a first metathesis zone or section 326. In the firstmetathesis zone 326, at least a portion of the residual quantity ofethylene from the dimerized stream is metathesized with at least aportion of the hexenes from the dimerized stream to produce a first orintermediate metathesis effluent stream in a line 328 comprising aquantity of butenes, including a quantity of 1-butenes, and a quantityof propylene. The first metathesis effluent stream in the line 328 canfurther comprise a quantity of pentenes.

The metathesis reaction in the first metathesis zone 326 can generallyemploy a catalyst such as described in detail above in conjunction withthe metathesis zone 30, illustrated in FIG. 1. The metathesis reactionin the first metathesis zone 326 can generally be carried out atconditions effective to result in the conversion of at least a portionof the hexenes from the dimerized stream 328 to propylene. For example,the metathesis of hexene with ethylene can, for example, be carried outin the vapor phase at a temperature in a range of about 300° C. to about350° C., and, typically, at about 330° C. The metathesis reaction cangenerally, typically carried out at a pressure of about 0.5 MPa (75psia) with a WHSV of 50 to 100.

The first metathesis effluent stream, or at least a portion thereof, ispassed via the line 328 to an isomerization zone 330. In theisomerization zone 330, at least a portion of the 1-butenes from thefirst metathesis effluent stream are isomerized to produce an isomerizedstream in a line 332 comprising a quantity of 2-butenes. The isomerizedstream in the line 332 can further include at least a portion of thequantity of pentenes and/or a portion of the quantity of propylene fromthe first metathesis effluent stream in the line 328. The isomerizationreaction can generally employ catalysts and be carried out as describedin detail above in conjunction with the isomerization zone 260, asillustrated in FIG. 3.

At least a portion of the quantity of 2-butenes from the isomerizedstream and a quantity of ethylene, such as from a portion of the firstprocess stream in the line 310, are introduced via lines 332 and 340,respectively, into a second metathesis zone or section 334 to produce asecond metathesis effluent stream in a line 342 comprising propylene. Inaccordance with certain embodiments, at least a portion of the quantityof pentenes from the isomerized stream in the line 332 are alsometathesized with ethylene in the second metathesis zone 334 to producean additional quantity of propylene and a quantity of 1-butenes whichcan desirably be recovered from the second metathesis effluent stream.

The metathesis reaction can generally employ catalysts such as describedin detail above in conjunction with the metathesis zone 34, asillustrated in FIG. 1. In accordance with certain embodiments, themetathesis of 2-butene with ethylene can, for example, be carried out inthe vapor phase at about 300° C. to about 350° C. and about 0.5 MPa (75psia) with a WHSV of 50 to 100 and a once-through conversion of about15%, depending on the ethylene to 2-butene ratio.

The second metathesis effluent stream, or at least a portion thereof, isintroduced via the line 342 into a metathesis fractionation zone 344wherein the second metathesis effluent stream is resolved, e.g.,fractionated, by conventional separation means into a propylene productstream 346 and a butenes fraction 348 generally composed of at least aportion of a residual quantity of butenes including, in accordance withcertain embodiments, a portion of the quantity of 1-butenes from thesecond metathesis effluent stream. The butenes fraction 348, or at leasta portion thereof, can be recycled back into the processing scheme 300such as, for example, by introducing the butenes fraction 348 into theisomerization zone 330 via a line 350. In embodiments wherein suchbutenes fraction 348 from the metathesis fractionation zone 344 isrecycled to the isomerization zone 330, a drag or purge stream 352 maybe provided to reduce the build-up of select hydrocarbon components suchas, for example, isobutene, in the process loop.

The second metathesis effluent stream in the line 342, or at least aportion thereof, can be further treated in the metathesis fractionationzone 344 to produce or result in the formation of an ethylene recyclestream in a line 354 generally composed of at least a portion of aresidual quantity of ethylene from the second metathesis effluent streamand which can be subsequently recycled to the dimerization zone 320 viathe line 354.

The second metathesis effluent stream in the line 342 can be furtherresolved in the metathesis fractionation zone 344, such as byconventional distillation methods, to produce a third process streamgenerally composed of C₅ and/or C₆ materials such as, for example,residual quantities of pentenes and/or hexenes from the secondmetathesis effluent stream, which can be introduced via a line 358 intothe oxygenate conversion effluent treatment zone 308 for furtherprocessing. Alternatively, the third process stream can be removed fromthe processing scheme 300. For example, at least a portion of the thirdprocess stream can be used for fuel.

The second metathesis effluent stream in the line 342 can be furtherresolved, e.g., fractionated, in the metathesis fractionation zone 344,such as by conventional distillation methods, to produce a heavyhydrocarbon purge stream 360. In practice, the heavy hydrocarbon purgestream 360, or a portion thereof, can be used as fuel. For example, forlocations in proximity to refineries, such materials or select portionsthereof can be blended into a gasoline pool. Additionally oralternatively, depending upon the specifications as to the olefincontent in a feed to a synthesis gas unit, the heavy hydrocarbon purgestream 360 or a portion thereof can be recycled to a front-end synthesisgas unit.

In accordance with certain embodiments, at least a portion of thebutenes, including a quantity of 1-butenes, from the second processstream in the line 312 can be isomerized in the isomerization zone 330to produce an additional quantity of 2-butenes. Additionally oralternatively, the processing scheme 300 can include a hydrogenationzone 362 wherein at least a portion of the quantity of diolefins fromthe second process stream in the line 312 can be hydrogenated to producea hydrogenation effluent stream in a line 364 comprising an additionalquantity of 1-butenes. In practice, the hydrogenation effluent stream,or at least a portion thereof, can be introduced into the isomerizationzone 330 via the line 364 to produce an additional quantity of2-butenes.

The invention illustratively disclosed herein suitably may be practicedin the absence of any element, part, step, component, or ingredientwhich is not specifically disclosed herein.

While in the foregoing detailed description this invention has beendescribed in relation to certain embodiments thereof, and many detailshave been set forth for the purpose of illustration, it will be apparentto those skilled in the art that the invention is susceptible toadditional embodiments and that certain of the details described hereincan be varied considerably without departing from the basic principlesof the invention.

1. A process for producing light olefins from an oxygenate-containingfeedstock, the process comprising: contacting the oxygenate-containingfeedstock in an oxygenate conversion reactor with an oxygenateconversion catalyst and at reaction conditions effective to convert theoxygenate-containing feedstock to an oxygenate conversion effluentstream comprising light olefins and C₄+hydrocarbons, wherein the lightolefins comprise ethylene and the C₄+hydrocarbons comprise a quantity ofbutenes; treating the oxygenate effluent stream in a separation zone andforming a first process stream comprising at least a portion of theethylene from the oxygenate conversion effluent stream; dimerizing atleast a portion of the ethylene from the first process stream in adimerization zone maintained with an ethylene to butene ratio from 1:1to 5:1 to produce a dimerized stream comprising a quantity of butenes;contacting at least a portion of quantity of butenes from the dimerizedstream with ethylene in a metathesis zone at effective conditions toproduce a metathesis effluent stream comprising propylene; andrecovering propylene from the metathesis effluent stream.
 2. The processof claim 1 wherein the oxygenate-containing feedstock is selected frommethanol, ethanol, dimethyl ether, diethyl ether, and combinationsthereof.
 3. The process of claim 1 wherein the treating stepadditionally forms a second process stream comprising at least a portionof the butenes including a quantity of 1-butenes from the oxygenateconversion effluent stream and wherein at least a portion of butenesfrom the second process stream is contacted with ethylene in themetathesis zone to produce propylene.
 4. The process of claim 3additionally comprising: isomerizing at least a portion of the quantityof 1-butenes from the second process stream in an isomerization zone toproduce an isomerized stream comprising a quantity of 2-butenes; andcontacting at least a portion of the quantity of 2-butenes from theisomerized stream with ethylene in the metathesis zone to producepropylene.
 5. The process of claim 1 wherein the metathesis effluentstream additionally comprises a quantity of ethylene and a quantity ofbutenes and wherein the metathesis effluent stream is separated in afractionation zone to form a propylene product stream, an ethylenerecycle stream and a butenes recycle stream.
 6. The process of claim 5additionally comprising: introducing at least a portion of the ethylenerecycle stream into the dimerization zone to produce an additionalquantity of butenes; and treating the butenes recycle stream by one ofintroducing at least a portion of the butenes recycle stream into themetathesis zone to produce an additional quantity of propylene orintroducing at least a portion of the butenes recycle stream to anisomerization zone to produce an additional quantity of 2-butenes. 7.The process of claim 1 wherein the dimerized stream additionallycomprises a quantity of 1-butenes and wherein at least a portion of thequantity of 1-butenes from the dimerized stream are isomerized in anisomerization zone to produce an isomerized stream comprising a quantityof 2-butenes.
 8. The process of claim 7 additionally comprising:contacting at least a portion of the 2-butenes from the isomerizedstream with ethylene in the metathesis zone to produce propylene.
 9. Theprocess of claim 1 additionally comprising: introducing a portion of theethylene from the first process stream into the metathesis zone.